Variable; A symbol for a number we don't know yet. It is usually a letter like x or y. Example: in x + 2 = 6, x is the variable. [mathsisfun.com] Q. Why is it called the variable? This seems to me to imply that its value varies. In the above example it seems to me to simply be the unknown term, not one that varies (x in the above example = 4, without any variance). Wouldn’t ‘unknown term’ be a more helpful label? * constant 1. A component of a relationship between variables that does not change its value, e.g. in y = ax + ... [Oxford dictionary of physics] Q. Is the c squared (9 x 10^16 m/s, or 90 quadrillion m/s) in E = mc^2 the constant (and E and m the variables)? Q. Is the g (10 m/s^2) in w = mg the constant (and w and m the variables)? If so, then I can see here why 'variable' is a helpful label as E and M vary according to the various examples of situations involving such phenomena [as does w and m]; but it doesn’t to me seem helpful when applied to the example of x + 2 = 6 (where the x, although unknown, does not vary; it’s always 4). Is 'variable' then a helpful term in physics but less so in maths?